This invention relates to semiconductor fabrication processes and, more particularly, to antireflective coatings and wiring line processes.
Integrated circuits (ICs) commonly are fabricated on a semiconductor wafer. The wafer typically is cut to form multiple semiconductor substrates or xe2x80x9cIC chips.xe2x80x9d Semiconductor devices are formed on the wafer. Although the label semiconductor is used, the devices are fabricated from various materials, including electrical conductors (e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon) and electrical non-conductors (e.g., silicon dioxide). The semiconductive silicon wafer is subjected to deposition, etching, planarizing and lithographic processes to achieve the many semiconductor devices. Aluminum and aluminum alloy metallization techniques are used to create contacts and interconnects among devices.
The advantages of manufacturing ICs with smaller individual circuit elements so that device density is increased are well known: electronic equipment becomes less bulky, reliability is improved by reducing the number of connections, assembly and packaging costs are minimized, and circuit speeds increase. Interconnect technology is increasingly a limitation in increasing circuit density for very large scale integration (VLSI) devices. In particular, achieving smaller line widths and smaller line spacings typically controls the number of devices that can occur on a semiconductor substrate. The ability to minimize the line widths and line spacings is limited by the lithographic processes, among others.
The term xe2x80x9clithographic processxe2x80x9d refers to a process in which a pattern is delineated in a layer of material sensitive to photons, electrons or ions. The principle is similar to that of a photo-camera in which an object is imaged on a photo-sensitive emulsion film. While with a photo-camera the xe2x80x9cfinal productxe2x80x9d is the printed image, the image in the semiconductor process context typically is an intermediate pattern which defines regions where material is deposited or removed. An antireflective material is desired during photolithographic processes to define areas to remain intact (e.g., positive exposure) or to be removed (e.g., negative exposure).
A current problem in patterning small dimensions for very large scale integrated (xe2x80x9cVLSIxe2x80x9d) circuits is notching of line edges, or, more generally, non-uniform line formation. Notching refers to grooves or other uneven cuts (i.e., xe2x80x9cnotchesxe2x80x9d) detracting from straight edges. Such notching has been less significant when patterning larger line widths and line spacing because the size of the notch relative to the line width or spacing was relatively small. At the smaller dimensions, however, notches can sever or significantly decrease a line width. A severed wiring line, for example, is an open circuit, and thus is defective. Reflection of light off metal lines during photolithography is one cause of notching.
FIGS. 1A and 1B show notched wiring lines 10, 13, and 15 formed on a semiconductor substrate 11, wherein notched wiring lines 10, 13, and 15 are separated by an insulative material 17 disposed over the semiconductor substrate 11. The notched wiring line 10 has a portion 12 along its length which is substantially thinner than other portions 14, 16. This thinner portion 12 is more susceptible to electromigration and stress migration. Electromigration is the transport of metal atoms by momentum exchange between electrons and metal ions. As the electrons move under the influence of a field, collisions between the electrons and ions transfer momentum to the ions. The ions, in turn, move in the direction of electron flow, leaving a vacancy in the wiring line metal. Over time, the vacancies accumulate, forming voids of non-conductivity in the metal. In effect, a defective open circuit occurs.
Stress migration is the atomic migration induced by thermal cycling of a wiring line between high operating temperatures and low environmental temperatures. The thermal cycling causes mechanical stresses on the thin lines, commonly referred to as creeping. The stress increases as the line width decreases. The predominant failure again is voiding in the metal, resulting in defective open circuits in wiring lines. Accordingly, uniform lines are desired to avoid notching and the resulting susceptibility to electromigration and stress migration. Such uniform lines are desired for increasingly smaller line widths and line spacings.
Another cause of migration failures in wiring lines is shrinkage during heat treatment processes. A conventional wiring line includes a conductor layer (e.g., aluminum) and an underlayer (e.g., titanium). During the fabrication processes which produce the wiring line and adjacent integrated circuitry, the titanium and aluminum undergoes a heat treatment process. During such process, adjacent regions of aluminum and titanium react to form a titanium aluminum compound between the titanium layer and the aluminum layer. In one application, the stable compound formed is TiAl3. In forming the compound, however, the aluminum layer shrinks, resulting in a 4-8% volume loss at the aluminum layer. This volume loss has the undesired effect of increasing stress in the aluminum layer (e.g., stress increases by 3-4 times for small line dimensions). As a result, voids are prone to form over time due to stress migration and electromigration. At small line dimensions, the metal may even go beyond its yield strength and break, leaving undesired voids after the heat treatment process. Accordingly, there is a need for a wiring line formation process which avoids undue stress upon the conductive layer.
There also is a need for an antireflective coating applied to a semiconductor substrate of a flat panel display device. In a cold cathode field emission display (xe2x80x9cFEDxe2x80x9d) device, the quality and sharpness of an illuminated pixel site of the display screen is dependent on the precise control of the electron emission from emitter sites that illuminate a particular pixel site. In forming a visual image, such as a number or letter, different groups of emitter sites must be cycled on or off to illuminate the appropriate pixel sites on the display screen. To form a desired image, electron emission may be initiated in the emitter sites for certain pixel sites while the adjacent pixel sites are held in an off condition. For a sharp image, it is important that those pixel sites that are required to be isolated remain in an off condition.
One factor that may cause an emitter site to emit electrons unexpectedly is the response of semiconductor junctions in the FED drive circuitry to photons. The adverse photons are generated by the luminescent display screen and/or by photons present in the environment (e.g., lights, sunshine). This may affect the junctions by changing their electrical characteristics. In some cases, this may cause an unwanted current to pass across the junction. The unwanted current may initiate electron emission from emitter sites of adjacent pixels. Such emission, in turn, may cause the adjacent pixel to illuminate when a dark pixel is desired. From a viewer""s perspective, illumination of undesired pixels may cause degraded or blurry images. Besides isolation and activation problems, light from the environment and display screen striking junctions on the substrate may cause other problems in addressing and regulating current flow to the emitter sites of the FED cell. Accordingly, there is a need to prevent given junctions from undesired exposure to photons.
According to the invention, titanium, aluminum and nitrogen (xe2x80x9cTixe2x80x94Alxe2x80x94Nxe2x80x9d) are deposited onto a semiconductor substrate area to serve as an antireflective coating. For wiring line fabrication processes, the Tixe2x80x94Alxe2x80x94N layer prevents unwanted reflection of photolithography light (i.e., photons) during fabrication. For field emission display devices (FEDs), the Tixe2x80x94Alxe2x80x94N layer prevents light originating at the display screen anode from penetrating transistor junctions that would hinder device operation.
According to one aspect of the invention, a wiring line (e.g., conductive path, interconnect) is formed by a titanium aluminum nitrogen (xe2x80x9cTixe2x80x94Alxe2x80x94Nxe2x80x9d) cap layer, an aluminum conductive layer, and a titanium aluminum underlayer. The Tixe2x80x94Alxe2x80x94N cap layer serves as the antireflective layer. The Tixe2x80x94Alxe2x80x94N layer overlays the aluminum layer which overlays the titanium-aluminum underlayer.
According to another aspect of the invention, the titanium-aluminum underlayer is deposited onto the substrate (or onto a barrier layer of the substrate, or onto a contact or via metallization layer) via sputtering or another physical vapor deposition (PVD) process. Thereafter, the aluminum layer is deposited, followed by the Tixe2x80x94Alxe2x80x94N layer. By applying a Tixe2x80x94Al underlayer, as opposed to a titanium-only underlayer, there is less shrinkage of the aluminum layer. For example, whereas an approximately 4000 angstrom aluminum layer over 500 angstroms of titanium would shrink by approximately 120 angstroms in certain heat treatment steps, starting with a Tixe2x80x94Al underlayer minimizes the shrinkage and volume loss of the aluminum layer. As a result, less stress is introduced to the aluminum layer during fabrication processes. The aluminum layer then is less susceptible to electromigration and less susceptible to stress migration.
According to one advantage of the invention, the Tixe2x80x94Alxe2x80x94N cap layer reduces reflectance by a factor of approximately 10 relative to a conventional titanium nitride cap layer for a typical photolithographic wavelength (e.g., 365 nanometers). As a result, notching is substantially reduced and electromigration resistance and stress migration resistance are improved. According to another advantage, the use of titanium-aluminum as an underlayer reduces stress at the aluminum layer, thereby improving electromigration resistance and stress migration resistance. Also, the use of a titanium-aluminum underlayer avoids the need for a separate sputter chamber when forming the Tixe2x80x94Alxe2x80x94N cap layer.
According to another aspect of the invention, an antireflective Tixe2x80x94Alxe2x80x94N layer is applied to a field emission display (xe2x80x9cFEDxe2x80x9d) device to protect drive circuitry junctions from photons generated in the environment or by the display screen anode. When fabricating the drive circuitry on a semiconductor substrate of an FED baseplate, an insulating layer typically is applied over the wiring lines and around the contacts. The antireflective Tixe2x80x94Alxe2x80x94N layer is applied over such insulating layer. A passivation layer or insulation layer then is applied over the Tixe2x80x94Alxe2x80x94N coating. Such a coating blocks photons from reaching the drive transistor junctions. An advantage of the antireflective coating is to protect the junctions from adverse changes to their electrical characteristics.